This application claims the priority of Korean Patent Application Nos. 2002-42762 and 2003-41060, filed on Jul. 20, 2002 and Jun. 24, 2003, respectively in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an apparatus and a method for adaptively enhancing the color of an image, and more particularly to an apparatus and a method for enhancing the color of an input image by adaptively changing the saturation of the input image.
2. Description of the Related Art
There have been some conventional techniques of enhancing the color of a color image. One is a method for image-dependent color saturation correction in a natural scene pictorial image disclosed in U.S. Pat. No. 5,450,217 (Xerox Corporation, 1995. 9). In this method, color saturation is enhanced in an HSV color space, which is considered as being capable of representing an image to have almost the same saturation as a recognized saturation of the image. However, this method can enhance the color saturation of certain images only, which have an average saturation of no higher than a certain threshold value. Therefore, in a case where this method is applied to consecutive images constituting a moving image and having a saturation of about the threshold value as average, it may cause flickering on a screen.
Another method is a method of automatically controlling color saturation in a video decoder using recursive algorithm disclosed in U.S. Pat. No. 6,188,788B1 (Texas Instruments Inc., 2001. 2). In this method, a YCbCr signal is obtained from a composite video signal and then compared with an ideal burst signal, thus deriving the ratio of the YCbCr signal to the ideal burst signal. Next, the color of an image is compensated for based upon the YCbCr signal-to-the ideal burst signal ratio by increasing the amplitude gain of a color signal using a recursive algorithm. However, this technique can be limitedly applied to video signals rather than all kinds of color images, and the variation of color difference signals may cause the variation of hues.
Still another method is a method for high-speed color saturation conversion disclosed in U.S. Pat. No. 5,315,694 (Toshiba Kabushiki Kaisha, 1994. 5). This method is based on the assumption that the amount by which the saturation of an RGB color signal of an input image is increased after converting the color signal of the input image into a signal in an HSI (H: hue, S: saturation, and I: intensity) color space is equal to the sum of the RGB vector of the color signal of the input image and the RGB vector obtained by the saturation enhancement in the RGB color space. In other words, the amount by which the saturation of a color signal is increased can be represented with vector in an RGB color space. Accordingly, it is possible to obtain an output RGB signal having an enhanced saturation by converting the amount by which the saturation of an original RGB signal is increased into a 3×3 matrix. According to this technique, a process of non-linearly converting a color space capable of representing an image with hue and saturation into another is unnecessary, and thus this technique is considered as high-speed saturation conversion. However, since this method collectively enhances the saturation of all images by the same amount, it is impossible to adaptively enhance the saturation of color signals in consideration of the characteristics of each input image.
The above-described conventional saturation enhancement techniques collectively rather than adaptively enhance the saturation of color signals of an input image by the same amount without considering the characteristics of portions of the input image, like portions representing a cloud, which are considered as low saturation regions. Accordingly, such low saturation regions of an input image may be represented with unnatural colors.